Trying to create a Python script that processes some data. I'm running this on QGIS 3.01 on MacOS 10.12.6. Having trouble with it as every time I run the below code, it completely crashes QGIS.

All I'm trying to do is run a series of processing algorithms. If I comment out the processing.run line, QGIS doesn't crash but also I can't do anything.

import processing
from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import QgsProcessingAlgorithm, QgsProcessing, QgsProcessingParameterFeatureSink

class testAlg(QgsProcessingAlgorithm):

    def tr(self, text):
        return QCoreApplication.translate('testalg', text)

    def createInstance(self):
        return type(self)()

    def group(self):
        return self.tr('Test')

    def groupId(self):
        return 'test'

    def __init__(self):

    def initAlgorithm(self, config=None):
        self.addParameter(QgsProcessingParameterFeatureSink(self.OUTPUT, self.tr('Output'), QgsProcessing.TypeVectorPolygon))

    def name(self):
        return 'testalg'

    def displayName(self):
        return self.tr('Test Algorithm')

    def processAlgorithm(self, parameters, context, feedback):
        # Commenting out the next line stops QGIS from crashing, but it means I can't actually do anything
        processing.run('qgis:fieldcalculator', {'INPUT':'/Users/myusername/Documents/temp/test.geojson','FIELD_NAME':'test','FIELD_TYPE':0,'FIELD_LENGTH':10,'FIELD_PRECISION':6,'NEW_FIELD':True,'FORMULA':'( "count" / sum(  "count" )) * 100','OUTPUT':'memory:'})

        return {}

Am I doing something horribly wrong in my script or is this a bug in QGIS 3?


Edit: By crash, I mean that QGIS freezes and becomes unresponsive. I have to Force Quit it and reopen it before I can do anything else.

Edit 2: This also freeze/crashes Windows 10.

  • I'm facing the same problem. I've made a processing scrip to perform K-Means classification with sklearn. The script works perfectly on my Linux installation, but crashes my QGIS on my Mac.
    – lcoandrade
    Sep 4, 2019 at 20:44

1 Answer 1


Use processing.run in this way:

# it'll be sensible to assing the result of processing to a variable
output = processing.run(
            'FORMULA':'( "count" / sum(  "count" )) * 100',
            feedback=feedback,          # add this line
            context=context)['OUTPUT']  # and this line # ['OUTPUT'] is optional

When I use in that way, it doesn't crash QGIS. Otherwise, QGIS freezes. (I tried in Windows 10)

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